1. Field of the Invention
The present invention generally relates to radio communications systems, and particularly relates to base station apparatuses, user apparatuses, and methods of allocating reference signal sequences.
2. Description of the Related Art
As a communications scheme to succeed W-CDMA (Wideband Code Division Multiple Access) and HSDPA, Long Term Evolution (LTE) is being studied in a W-CDMA standardization body called 3GPP. Moreover, as radio access schemes, OFDM is being considered for downlink, while SC-FDMA (Single-Carrier Frequency Division Multiple Access) is being considered for uplink (see Non-patent document 1, for example).
The OFDM, which is a scheme for dividing a frequency band into multiple narrow frequency bands (sub-carriers) and overlaying data onto the respective frequency bands for transmission, densely arranges the sub-carriers on the frequency axis such that one sub-carrier partially overlaps another sub-carrier without their interfering with each other, making it possible to achieve high-speed transmission and to improve frequency utilization efficiency
The SC-FDMA is a transmission scheme which divides a frequency bandwidth and transmits using different frequency bands among multiple terminals to make it possible to reduce interference between the terminals. The SC-FDMA, which features a reduced variation in transmission power, makes it possible to achieve wide coverage as well as low power consumption of the terminals.
A reference signal for uplink in E-UTRA refers to a pilot channel, and is used for synchronizing, channel estimating for coherent detection, and measurement of received SINR at the time of power control. The reference signal is a known transmission signal at the receiver, is embedded in each sub-frame, and is received at the base station.
In the W-CDMA, a signal sequence used for the reference signal (pilot channel) is a user-specific PN sequence, or, more precisely, a sequence such that long-period Gold and orthogonal sequences are multiplied. Moreover, many types of sequences may be created due to their great lengths. However, there is a problem that the channel estimation accuracy is reduced as the PN sequence is not superior in the correlation characteristics. In other words, when the pilot channel is transmitted, interference with pilot channels of other users becomes large. Moreover, delay waves in a multi-path cause a large auto-correlation problem with the pilot channel sequence. The W-CDMA provides for a simple reception process such as RAKE, while E-UTRA is designed based on the premise that highly-accurate channel estimation means such as an equalizer is used to suppress multi-path interference. Thus, in E-UTRA, a CAZAC (Constant amplitude and zero auto-correlation) sequence is used in lieu of the user-specific PN sequence (see Non-Patent document 2).
The CAZAC sequence, which is superior in code auto-correlation and cross-correlation characteristics, makes it possible to achieve highly-accurate channel estimation, allowing a significantly-improved demodulation accuracy in comparison with a case of using the PN sequence. The CAZAC sequence, having small signal amplitude variations in both frequency and time domains, is relatively flat. The PN has a large amplitude variation in the frequency domain. Thus, the CAZAC sequence may be used to make it possible to accurately perform channel estimation using an equalizer. Moreover, the CAZAC sequence makes it possible to suppress the effect of the multi-path to a small level since the auto-correlation of the transmitted sequence becomes zero.
However, the CAZAC sequence has the following problems. A small number of sequences: as it is not possible to make it be a user-specific sequence, cell repetition is needed for the sequences. The number of sequences becomes small when the transmission bandwidth is small, particularly in the SC-FDMA. When the transmission bandwidth is small in the SC-FDMA, in particular, the symbol rate becomes small, so that the length of the CAZAC sequence is reduced. In other words, in E-UTRA, the reference signal is inserted in time multiplexing. Thus, when the transmission bandwidth is small, the symbol rate becomes low, so that the sequence length becomes small. The number of sequences becomes the same as the length of the sequences. For example, for a bandwidth of 180 kHz and the number of symbols of 12, it is not possible to make the sequences user-specific, so that 12 sequences must be periodically allocated in the same cell so that they don't overlap. The cross-correlations between the CAZAC sequences of different sequence lengths have a relatively large variance depending on the combination. Thus, when the cross-correlation is large, the channel estimation accuracy is reduced.
Next, the SC-FDMA used for uplink radio access in E-UTRA is described with reference to FIG. 1. The frequency bandwidth available for use by a system is divided into multiple resource blocks, each of which includes at least one sub-carrier. At least one resource block is allocated to user equipment (UE). In frequency scheduling, resource blocks are preferentially allocated to terminals with good channel conditions depending on received signal quality or channel condition information (CQI) per resource block of a downlink pilot reported from a user apparatus. Moreover, frequency hopping may be applied which changes a frequency block available for use depending on a predetermined frequency hopping pattern.
In FIG. 1, different hatchings indicate time/frequency resources allocated to different users. UE2, which is allocated a larger bandwidth, is going to be allocated a small bandwidth in the following sub-frame. Different frequency bandwidths are allocated to the users so that there is no overlap.
In the SC-FDMA, the users in a cell transmit using different time and frequency resources. In this way, the users within the cell are made orthogonal. A minimum unit for the time and frequency resources is called a resource unit (RU). In the SC-FDMA, successive frequencies are allocated to achieve low PAPR (peak-to-average power ratio) single-carrier transmission. In the SC-FDMA, the base station scheduler determines the time and frequency resources to be allocated based on a propagation state of each user, and QoS (Quality of Service) of data to be sent. Here, the QoS includes a data rate, a predetermined error rate, and a delay. In this way, time and frequency resources may be allocated to each user with a good propagation condition to increase throughput.
Each of the base station apparatuses individually performs time and frequency resource allocations. Thus, a bandwidth allocated in one cell may overlap with a part of a bandwidth allocated in an adjacent cell. In this way, when there is an overlap with the part of the bandwidth allocated in the adjacent cell, interference occurs, leading to degradation across the cells.
Next, a reference signal in the uplink SC-FDMA is described with reference to FIG. 2. FIG. 2 shows an exemplary frame configuration.
The length of a TTI packet called a sub-frame is 1 ms. the sub-frame includes 14 blocks to undergo FFT, two of which are used for transmitting the reference signal and the remaining 12 of which are used for transmitting data.
The reference signal undergoes time-division multiplexing (TDM) with the data channel. The transmission bandwidth varies dynamically according to instructions from the base station apparatus through frequency scheduling. Thus, the lower the transmission bandwidth the lower the symbol rate, so that the sequence length of the reference signal transmitted in a fixed time length becomes small, while the larger the transmission bandwidth the higher the symbol rate, so that the sequence length of the reference signal transmitted in the fixed time length becomes large. For a narrow bandwidth (for example, when the reference signal is transmitted as one resource unit, or 12 sub-carriers, or 180 kHz), the number of symbols becomes 12. Thus, the sequence length becomes approximately 12 and so does the number of sequences. On the other hand, for a wide bandwidth (for example, when the reference signal is transmitted as 25 resource units, or 300 sub-carriers, or 4.5 MHz), the number of symbols becomes 300. Thus, the length of sequence becomes approximately 300 and so does the number of sequences.
Non-Patent Document 1
3GPP TR 25.814 (V7.0.0), “Physical Layer Aspects for Evolved UTRA,” June 2006
Non-Patent Document 2
3GPP TS 36.211 (V1.0.0), March 2007